In recent years, water-absorbent resins are widely utilized for various uses, such as sanitary articles (e.g. disposable diapers, sanitary napkins, and incontinent articles for adult) and water-retaining agents for soil, and are mass-produced and consumed.
Particularly, in uses of sanitary articles such as disposable diapers, sanitary napkins, and incontinent articles for adult, the amount of the water-absorbent resins as used tends to increase, and the amount of a pulp fiber as used tends to decrease, in order to thin the resultant articles. The water-absorbent resins are desired to have large absorption capacity under a load. On the one hand, desired are low-cost water-absorbent resins because the amount of the water-absorbent resins as used per sheet of sanitary articles is large. Therefore, it is desired that: the energy consumption and the amount of discharged materials are decreased in production lines of water-absorbent resins, and thereby a reasonable production process is established.
In order to decrease costs for improving a ratio of performance of the water-absorbent resins to their costs, various attempts (e.g. a method which involves carrying out polymerization in a high monomer concentration; and a polymerization method which involves initiating polymerization in high temperature, evaporating water by heat of polymerization or heating, and obtaining a dried water-absorbent resin at a stroke) have hitherto been made as a method for carrying out aqueous solution polymerization of a monomer component that is converted to water-absorbent resins by the polymerization.
In gazettes of JP-A-071907/1983 (Arakawa Chemical Industries. Ltd.) and JP-A-018712/1984 (Arakawa Chemical Industries. Ltd.), disclosed is a method which involves polymerizing an aqueous acrylate salt solution having a high concentration of higher than 55 weight %, and obtaining a dried solid water-absorbent resin at a stroke. In a specification of U.S. Pat. No. 4,985,518 (American Colloid), disclosed is a method which involves polymerizing an aqueous acrylate salt solution having a high concentration of higher than 30 weight %, and obtaining a dried solid water-absorbent resin at a stroke. In a gazette of JP-A-058208/1980 (Kitani), disclosed is a method that involves carrying out polymerization without using a crosslinking agent in the polymerization temperature range of 106 to 160° C. As is shown in its example, it is disclosed that a dried solid having a low water content is obtained when the polymerization is completed. In a gazette of JP-A-318022/1989 (Mitsubishi Petrochemical Co., Ltd.), disclosed is a method which involves polymerizing an aqueous solution including a monomer having a neutralization ratio of 20 to 50 mol % in an amount of 45 to 80 weight %, and obtaining a polymerized product nearly in a dry state. However, these methods have demerits such that the extractable content of formed water-absorbent resins is relatively large in comparison with their absorption capacity.
In addition, in gazettes of JP-A-147512/1980 (Sumitomo Chemical Co., Ltd.), JP-A-147809/1981 (Sumitomo Chemical Co., Ltd.), JP-A-275607/1988 (Sanyo Chemical Industries, Ltd.), and JP-A-275608/1988 (Sanyo Chemical Industries, Ltd.), it is disclosed that: a dried product of water-absorbent resin is obtained at a stroke by supplying an aqueous monomer solution onto a heated rotation drum, and then scratching and collecting the resultant product. In a gazette of JP-A-165610/1989 (Rohm and Haas Company), it is disclosed that a substantially dried solid water-absorbent resin is also obtained by supplying an aqueous monomer solution onto a heated face in the nearly same way. However, these methods also have demerits such that the extractable content of formed water-absorbent resins is relatively large in comparison with their absorption capacity.
In addition, in a gazette of JP-A-215801/1990 (Mitsubishi Petrochemical Co., Ltd.), it is disclosed that: the heat of neutralization of a monomer is utilized, and then the polymerization is carried out by spraying a heated aqueous monomer solution in a gas phase. However, the control of the polymerization is thought to be difficult because the polymerization is completed within about 3 seconds.
The above prior arts were techniques as disclosed before 1990, but each has demerits. Therefore, it seems that they are not carried out actually.
Thereafter, disclosed are techniques for enhancing performance in order to improve a ratio of performance of the water-absorbent resins to their costs. In gazettes of JP-A-175319/1992 (Sanyo Chemical Industries, Ltd.) and JP-A-181005/1999 (Nippon Shokubai Co., Ltd.), it is disclosed that: an attempt is made to obtain water-absorbent resins having high performance by initiating polymerization at a low temperature, mildly carrying out the polymerization while being cooled, and thereby suppressing the peak temperature to not higher than about 90° C. In a gazette of JP-A-228604/1999 (Nippon Shokubai Co., Ltd.), it is disclosed that: an attempt is made to obtain water-absorbent resins having high performance by also initiating polymerization at a low temperature, mildly carrying out the polymerization while being cooled, and thereby suppressing the peak temperature to not higher than about 95° C. or controlling the amount of the solid component concentration as increased in the range of 0.2 to 10 weight %. In addition, in WO 01/38402A1 (BASF), it is disclosed that: an attempt is made to obtain water-absorbent resins having high performance by also initiating polymerization at a low temperature, and cooling from a polymerization vessel wall and discharging the resultant polymerized gel in order to suppress the consumption of heat of reaction (e.g. in order to suppress the peak temperature to not higher than about 100° C.). In agazette of JP-A-067404/1997 (BASF) and a specification of U.S. Pat. No. 6,187,828 (BASF), disclosed is a method that involves initiating polymerization at a low temperature in a cylindrical polymerization vessel, and then adiabatically carrying out the polymerization. However, the cooling is not carried out, and therefore the method has demerits such that the concentration of the aqueous monomer solution cannot be increased, namely, demerits such that the residence time is prolonged (a few hours). Both of these are carried out at the sacrifice of productivity, and therefore it is inevitable to need great costs.
In addition, recently, in Journal of Applied Polymer Science, Vol. 74, 119 to 124 (1999), report was “An Efficient Preparation Method for Superabsorbent Polymers” (Chen. Zhao). This proposes a low-cost polymerization method which involves charging an aqueous solution having a monomer concentration of 43.6% and an initiator in a stainless-steel-made petri dish, immersing the petri dish in a water bath of 70 or 80° C., and then carrying out polymerization, but the method has not reached an industrially useful level.
In addition, in a gazette of JP-A-045812/1998 (Selisui Plastics Co., Ltd.), it is disclosed that an attempt is made to prevent bumping, to improve emission of vapor, and to lower the water content of formed gel, by adding a short fiber to an aqueous monomer solution. However, it has demerits of using the valuable short fiber that does not contribute to water absorption.